Shrinkage-compensating concrete

ABSTRACT

A shrinkage compensating concrete does not require restraint. The expansive forces developed during hydration compensate for concrete shrinkage, obviating the need for any added internal or external restraint element. Using this new shrinkage compensating concrete, substantially crack-free slabs may be built without using restraining steel bars, fibers, or other separate restraining element. The shrinkage compensating concrete includes a cement that develops internal expansive forces that never exceed the tensile strength of the concrete, such that the internal expansion compensates for the concrete shrinkage. The expansive cement may be an ASTMS, M or S cement, or other expansive cements may also be used.

PRIORITY CLAIM

This Application is a Continuation of application Ser. No. 13/840,796filed Mar. 15, 2013 and now pending, which claims priority to U.S.Patent Application No. 61/694,175 filed Aug. 28, 2012, both incorporatedherein by reference.

BACKGROUND OF THE INVENTION

The field of the invention is improved shrinkage compensating concrete.As is well known, traditional concrete tends to shrink as it dries orcures. This shrinkage occurs with loss of water as the concrete dries.The drying shrinkage creates tensile stresses in the concrete. Sinceconcrete generally has low tensile strength, shrinkage stresses oftencause cracking.

To avoid or reduce cracking caused by shrinkage, various expansiveconcretes have been used. See for example Klein, U.S. Pat. No.3,251,701, Rice, U.S. Pat. No. 4,419,136, and Rice U.S. Pat. No.5,846,316, each incorporated herein by reference. These and othershrinkage-compensating concretes include an expansive component orcement. The expansive cement generally is a hydraulic cement that itselfincludes an expansive component that expands during hydration. Theexpansive cement causes the concrete to expand slightly as it dries,which helps to offset or compensate for the shrinkage associated withdrying. As a result, shrinkage and resulting tensile stresses in theconcrete are reduced or eliminated, along with the cracking resultingfrom those stresses.

The tensile strength of concrete increases over time, using an expansivecement can also help reduce shrinkage cracking by reducing the tensilestresses, until the concrete acquires sufficient tensile strength tobetter withstand the tensile stresses without cracking. After theconcrete has expanded, subsequent drying shrinkage will reduce theexpansive stresses. Ideally though, a residual compression may remain inthe concrete indefinitely, thereby eliminating shrinkage cracking.

Shrinkage compensating concrete conventionally requires a restraintelement to prevent the concrete from over-expanding, which leads tocracking, crumbling and/or spalling. The restraint element may beexternal, such as other building structures, or temporary externalconstruction plates or bars, such as described for example in YtterbergU.S. Patent Application No. 2009/0071086. More often though, therestraint element is provided internally using steel rods, bars, mesh orfibers embedded into the shrinkage compensating concrete.

Since the advent of shrinking compensating concretes, for examplebeginning with Klein U.S. Pat. No. 3,251,701 as far back as 1961, orearlier, the industry and engineering convention has been that shrinkagecompensating concretes must be restrained to achieve desiredperformance. Indeed, the relevant material standard, ASTM C 845 evendefines shrinkage compensating concrete as a concrete that is internallyrestrained with resilient (e.g., steel) reinforcing and made withexpansive cement with induces compressive stress in the concrete thatapproximately offsets tensile stresses that result from dryingshrinkage. The minimum percentage of steel for restraint is 0.15% of thecross-sectional area. This is the restraint used in ASTM 878 StandardTest Method for Restrained Expansion of Shrinkage-Compensating Concrete.Correspondingly, various building codes specify that structures madewith shrinkage compensating concrete must have a minimum amount ofrestraint, typically specified as a minimum amount of steel restrainingrods. Thus, for over 50 years all known shrinkage compensating concretestructures have used added restraining elements.

An ideal shrinkage compensating concrete would have sufficientself-restraint to avoid cracking, without use of any steel bars, steelor non-metal fibers, or any other added restraining element. However, nosuch shrinkage compensating concrete has yet been realized.

SUMMARY OF THE INVENTION

A new shrinkage compensating concrete not requiring restraint has nowbeen invented. In this new shrinkage compensating concrete, theexpansive forces developed during hydration compensate for concreteshrinkage, obviating the need for any added internal or externalrestraint element. Using this new shrinkage compensating concrete,substantially crack-free slabs may be built without using restrainingsteel bars, fibers, or other separate restraining element.

In one aspect, a shrinkage compensating concrete includes a cement thatdevelops internal expansive forces that never exceed the tensilestrength of the concrete, such that the internal expansion compensatesfor the concrete shrinkage, resulting in a shrinkage-compensating slabor other structure without the need for internal or externalreinforcement. The expansive cement may be an ASTMS, M or S cement, orother expansive cements may also be used. Typically the expansive cementmay include calcium sulfoaluminate or any other oxide or sulfate thatexpands upon hydration.

Other and further objects and advantages will become apparent to personsskilled in the art from the following detailed description, which isprovided by way of example, and not as limitations on the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph of typical shrinkage as a function of time for aself-leveling hydraulic cement composition, with and without anexpansive additive.

FIG. 2 is a graph of expansion of a cement mortar composition withvarious amounts of expansive additives.

DETAILED DESCRIPTION Definitions

Expansive cement: a cement that when mixed with water forms a pastethat, after setting, tends to increase in volume to significantlygreater degree than Portland cement paste. ACI 223-98.

Shrinkage compensating cement: A cement that, when mixed with water,produces a paste that, after setting, increases in volume to asignificantly greater degree than does Portland cement paste. ACI223-98.

Shrinkage compensating concrete: The ASTM standard specification forexpansive hydraulic cement (ASTM C 845) defines shrinkage compensatingconcrete as a concrete that is internally restrained with resilientreinforcing and made with expansive cement with induces both compressivestress in the concrete and positive steel strain that approximatelyoffsets tensile stresses induced by drying shrinkage.

Several methods may be used to measure the expansive properties ofconcrete (See Rice, U.S. Pat. No. 3,779,085). The restrained expansionof mortar is generally measured using ASTM C 806: “Test Method forRestrained Expansion of Expansive Cement Mortar”. The compressivestrength of the expansive cement is measured using ASTM Test Method C109/C109M, except that a water-cement ratio of 0.50 shall be used, thespecimens shall be covered with a polyethylene sheet or other suitablematerial for preventing loss or gain of moisture at the surface of thespecimens during the moist storage period in the molds, and thespecimens shall remain in the molds for 3 days.

The scope of ASTM C 806 covers the determination of length changes ofexpansive mortar, while under restraint, due to the development ofinternal forces resulting from hydration of the cement. The apparatusused is as follows:

Molds: The molds for casting test specimens, when used in conjunctionwith the restraining cage described below, shall provide for formingeither 2 by 2 by 10 in. prisms having a 10-in. gage length, or 50 by 50by 250 mm prisms having a 250 mm gage length. The molds shall otherwiseconform to the requirements of Practice C 490, except that the cage studholder, gage stud and spacer screws described in that specificationshall not be used.

Restraining cage: The cage consists of a threaded steel rod with steelend plates held in place by nuts. The rod shall be provided with capnuts for the prevention of corrosion. The rod shall conform tospecification A 307 grade A steel.

The method for measuring the restrained expansion in concrete is ASTM C878 “Restrained Expansion of Shrinkage Compensating Concrete”. The scopeof ASTM C 878 covers the determination of the expansion of concrete madewith shrinkage compensating cement. Its significance and use are asfollows:

Since the potential for expansion, under conditions of controlledrestraint, of concrete made with shrinkage compensating cement cannotalways be satisfactorily predicted from tests of mortars made inaccordance with Test Method C 806, a need has been recognized for a testmethod in which concrete specimens are tested. This test method can alsobe adapted readily to studies of expansion involving degrees ofrestraint, comparison of cements, effects of cement contents,aggregates, mixture proportions, schedules or environmental treatmentsthat differ from the standards procedures prescribed by this testmethod. The test restrains expansion using internal steel.

External restraint: A restraint element external of the concreteconventionally used to restrain shrinkage compensating concrete.External restraint typically may be a surrounding structure, such as apreviously existing concrete slab or a wall, that prevents a newlypoured shrinkage compensating concrete from expanding. Pouring forms,such as plates, pipes, plywood, etc. used to simply confine the concreteare not external restraint. These types of forms confine, but do notrestrain, the concrete.

Internal restraint: A restraint element within the concrete used torestrain the expansion of the concrete. Typically these are steel barsor rods, or steel or polymer fibers.

Substantial restraint in a slab. Substantial restraint for the purposesof this invention is defined in ACI 223R-10, section 5.2.2 as a ratio ofsteel reinforcement area to gross concrete area of 0.15%. Consequently,a slab built with a ratio of 0.015% or less shall be considered aslacking substantial restraint.

Discussion

The inventors have discovered that shrinkage compensating concrete maybe used without any restraint element, while still largely avoidingcracking. Using the concrete of the invention, tensile stresses createdby shrinkage remain smaller than the tensile strength at any given time,in the absence of restraint, thereby preventing cracking. Two examplesare provided below: a slab on grade (Example 1) and a self-levelingcomposition (Example 2).

EXAMPLE 1 Restraint-Free Shrinkage Compensating Concrete

A new shrinkage compensating concrete may be used with no internal orexternal restraining elements, and will still resist shrinkage crackingas well as expansive concrete structures using restraining elements,such as steel bars or steel or non-metal fibers. This unrestrainedshrinkage compensating concrete may be made in various ways. One exampleis made with the mix proportions in Table 1.

TABLE 1 BATCH ABS. WT VOL. SPEC. MATERIAL AMOUNT SOURCE (LB) FT³ GRAVCement Type ASTM C150 382 1.94 3.15 II/V Cement Type K CTS 123 0..633.15 Komponent ® Water 33.3 gallons 277.4 4.45 1.00 No. 3 Aggregate 35%Vuln. Bg Rk 1141 6.9 2.65 Crk SV No. 4 Aggregate 26% Vuln. Bg Rk 8435.12 2.65 Crk SV Concrete Sand 39% Vuln. Bg Rk 1272 7.69 2.65 Crk SVC494 Type A 4.0 oz/cwt Eucon NW 20.2 WR C + P oz/cubic yard MaterialTotals 4043.2 27.00 Air Content  1% 0.27 Plastic Unit 149.7 pcf WeightKomponent® is an expansive material per ASTM Type K available from CTSCement Mfg. Co. Cypress, Calif.

Table 2 shows the aggregate gradation used for this concrete.

TABLE 2 Size (mm) 4.75 2.36 1.18 0.6 0.3 0.15 0.075 37.5 25 19 12.5 9.5No. 4 No. 8 No. 16 No. 30 No. 50 No. 100 No. 200 FM No. 3 100 95 69 4216 4 0 0 0 0 0 0 7.11 Agg. No. 4 100 100 91 16 3 0 0 0 0 0 5.90 Agg.Concrete 100 98 83 67 45 21 8 2 2.78 Sand Comb. 100 98 89 80 68 44 33 2618 8 3 1 5.11 Grad.

The concrete in Example 1 was cast as a 6 inch slab on ground. Twolayers of thick plastic were placed between the sub-base and theconcrete slab. The expansion in the concrete as measured by C 878 barswas 0.04% in seven days. Concrete strength by ASTM C 39 was 2,314 psi at7 days and 2,632 psi at 28 days. The mortar expansion as measured byASTM C 806 was 0.1830% at 7 days for 24 wt % Komponent®. This expansionalso places the cement of the outside the bounds defined by ASTM C 845.An inspection of the slab after five months found the slab to be crackfree.

EXAMPLE 2 Self-Leveling Floors

The concept described above can be extended to construction materialsused as underlayment, self-leveling floors, and/or toppings. Aself-leveling hydraulic cement-based topping mix usually exhibits highflow characteristics. It is typically used to create a flat and smoothsurface with a compressive strength similar to or higher than that oftraditional concrete prior to installing interior floor coverings. Whenit is poured, it has a viscosity similar to pancake batter. The lowviscosity is obtained through the addition of polymers and/or largeamounts of water. Since all of this water is not needed in the hydrationof the cement, its evaporation can lead to drying shrinkage andcracking.

The addition of controlled amounts of expansive additives will adjustthe expansion of the floor topping to minimize drying shrinkage andresult in a near-zero dimensional change during drying. As a result, theself-leveling floor is essentially crack-free.

FIG. 1 shows the typical shrinkage as a function of time for aself-leveling hydraulic cement composition (TRU®, available from CTSCement Manufacturing Co, Cypress Calif.) with and without an expansiveadditive (Komponent® ASTM Type K expansive cement, available from CTSCement Manufacturing Co, Cypress Calif.). The measurement was made usinga shrinkage cone apparatus fitted with a laser beam measurement device(Schleibinger Testing Systems). The advantage of the shrinkage conetechnique, compared to the traditional ASTM C 878 bar is that theshrinkage is tested in the absence of metallic restraint. After 3 days,the shrinkage of the self-leveling composition was 0.005 in. withoutexpansive additive, and 0.0027 in. (or about half that of the unmodifiedcomposition) with 10% of the expansive additive Komponent®. With 12% ofthe additive, the material showed an expansion of 0.002 in. and 0.004in. with a 14% addition of Komponent ASTM Type K expansive cement.

EXAMPLE 3 Restraint-Free, Shrinkage-Compensated Mortar

Mortar can also be modified to exhibit crack-free expansion compensatedto zero-shrinkage. Length change in such materials is usually testedusing ASTM C157 and ASTM C596 standards. Is this example, the specimenswere removed from the mold 30 minutes after final set and a firstreading was taken. They were then stored in lime-saturated water for 7days. Length change measurements were taken every 30 minutes for 3 hoursafter the initial reading, then daily for the seven days. After 7 days,specimens were taken out of lime water, and stored in air (73±3° F.,50±4% humidity).

FIG. 2 shows that the early expansion caused by additives is able tocompensate the subsequent shrinkage overtime-in the absence ofrestraint, which is a departure from the prior art. FIG. 2 showsshrinkage for the base material (CSA cement) reduced to near zero withthe use of expansive additives (such as, but not limited to CalciumSulfate and hydrated lime).

Mix Proportions Mix 1 2 3 CSA Cement 33.33% 31.83% 31.50% ASTM C77866.67% 66.67% 66.67% Silica Sand Hydrated Lime  0.00%  0.20%  0.20%Calcium Sulfate  0.00%  1.30%  1.63% Water to Cement  0.47  0.47  0.47Ratio

As described, a concrete may include an expansive cement such that theexpansive forces developed during hydration compensate concreteshrinkage, obviating the need for internal or external metallicrestraint.

A concrete may include an expansive cement such that the expansiveforces developed during its hydration compensate concrete shrinkage,obviating the need for restraint and resulting in a substantiallycrack-free slab.

A concrete may include a cement developing internal expansive forces atall times smaller than its tensile strength, such that the internalexpansion compensates concrete shrinkage, resulting in ashrinkage-compensating slab without the need for internal reinforcement.The expansive cement may be a Type K cement. The expansive cement mayinclude calcium sulfoaluminate or any other oxide or sulfate expandingupon hydration.

A shrinkage-compensating concrete or mortar may be providedsubstantially without internal restraint in which the tensile strengthof the concrete or mortar exceeds the expansive forces in the concrete

A self-leveling hydraulic cement-based topping composition may beprovided such that the internal expansion compensates shrinkage,resulting in a shrinkage-compensating topping without the need forinternal reinforcement.

A self-leveling hydraulic cement based topping composition may includean expansive agent causing internal expansion to compensate forshrinkage while remaining below the tensile strength, resulting in anessentially crack-free self-leveling floor. The expansive agent may be acalcium sulfoaluminate such as Komponent® ASTM Type K cement.

A method is provided for placing essentially crack-free slabs ofconcrete in which an expansive cement is mixed with hydraulic cement.The expansion compensates for shrinkage, so that the need for internalor external reinforcement is obviated.

A method is provided for placing essentially crack-free self-levelingfloors wherein an expansive compound is mixed with a self-levelingtopping composition. The expansion compensates for shrinkage so that theneed for internal or external reinforcement is obviated.

Thus, a novel concrete and method has been shown and described. Variouschanges may of course be made without departing from the spirit andscope of the invention. The invention, therefore, should not be limitedexcept by the following claims and their equivalents.

1. A shrinkage-compensated crack-free concrete structure, comprising: ahydraulic cement, an expansive additive, aggregate and water; and withthe structure having no internal restraint and no external restraint,and the structure having sufficient self-restraint wherein the structureremains free of cracks via expansive forces developed during hydrationcompensating for concrete shrinkage and with the length of the structureover time never shrinking below its original length.
 2. The concretestructure of claim 1 having internal expansive forces which at all timesare lower than its tensile strength, such that the internal expansioncompensates concrete shrinkage, resulting in a shrinkage-compensationwithout the need for reinforcement.
 3. The concrete structure of claim 1wherein the expansive additive is an ASTM Type K cement.
 4. Ashrinkage-compensated crack-free concrete structure, comprising: anexpansive hydraulic cement, aggregate and water; with the structure freeof cracks and having no metallic internal restraint.
 5. The concretestructure of claim 4 wherein the structure comprises a slab.
 6. Theconcrete structure of claim 4 wherein the structure has no internalrestraint and no external restraint.
 7. The concrete structure of claim4 wherein the concrete meets ASTM C-845 without using internal metallicrestraint.
 8. The concrete structure of claim 4 wherein the expansivehydraulic cement is an ASTM Type K cement.